Method for production of PZT-based ceramics having a slow sintering temperature

ABSTRACT

A method for manufacturing a PZT-based low-sintering piezoelectric ceramic material, the ions to be added as starting compounds being added as powdered oxides and/or powdered carbonates, mixed together, and then calcined to form the piezoelectric ceramic material. After calcining the starting compounds, lithium in ionic form is added to the mixture in an amount of 0.01 to 0.1 wt. % in relation to the weight of the PZT ceramic.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing PZT-basedceramics having a low sintering temperature.

DESCRIPTION OF RELATED ART

Ceramic, in particular piezoceramic, components, include several, inparticular many, layers (multilayer components); they are usable, forexample, as actuators in piezostacks due to the fact that a low-inertiamechanical deflection of a relatively high force is achieved by voltagetriggering, or are usable as bending elements due to the fact thatvoltage triggering causes a high mechanical deflection of low force, orthey allow high electric voltages to be generated, or are used inappropriate devices for detecting mechanical vibrations or forgenerating acoustic vibrations.

Previous technical approaches are predominantly based on ceramiccompounds of a Perovskite-type structure of the general formula ABO₃,the piezoelectric properties being manifested in the ferroelectricstate. Lead-zirconate titanate ceramics Pb(Zr_(1-x)Ti_(x))O₃(PZT),modified by certain additives,i.e., PZT-based ceramics, have proven tobe particularly advantageous. Noble metal internal electrodes appliedusing serigraphic methods are located between ceramic layersmanufactured using typical ceramic foil technology. When appropriateadditives or dopants are used, PZT-based piezoelectric ceramics have anexcellent combination of properties, such as high temperatureresistance, a high piezoelectric charge constant, high Curietemperature, low dielectric constant, and low coercive field intensity.

PZT-based piezoelectric ceramics are sintered together with copper- orAg/Pd-based electrode compound layers in a cofiring process to form anelectromechanical multilayer component. If Ag/Pd mixtures having asilver content considerably higher than 70 wt. % or even pure silverinternal electrodes, which are less expensive than platinum-basedmaterials or internal electrodes having a higher platinum content, areused for forming such multilayer components, a lower sinteringtemperature is desirable, because silver internal electrodes have arelatively low melting point of approximately 960° C.

Low-sintering PZT compositions, such as Zr-, Nb-, and Ti(PZ-PN-PT)-based ternary systems, are known. These compositions oftenhave a high PbO excess as a sintering aid, which has the disadvantagethat it strongly interacts with internal electrodes of Ag, AgPd, orcopper, for example, in the application as piezoelectric multilayeractuators. A high PbO excess has, in general, a disadvantageous effect,because it has the tendency to form low-melting metal (Ag, Pd, Cu)-Pballoys. In extreme cases, the internal electrodes run out or adhere bymelting. In principle, interaction between the PZT ceramic and theinternal electrode material has also been observed in ternary systems orbinary systems having complex doping (regarding content and number ofdopants). A high PbO excess in combination with ternary systems orbinary systems having complex doping is therefore consideredparticularly critical.

Furthermore, sintering aids such as additives of barium in the form ofBa(Cu_(0,5)W_(0,5))O₃, vanadium in the form of V₂O₅, or glassescontaining boron, bismuth and cadmium (BBC) are known for lowering thesintering temperature. Lithium additives in the form of Li₂CO₃ or LiNO₃are also known, but they have been unable to achieve the desired effectof lowering the sintering temperature while preserving or even improvingthe electromechanical properties. Instead, the opposite occurred, i.e.,the properties deteriorated in comparison with PZT ceramics without theaddition of sintering aids. One reason for this is that both the amountof sintering aids and the type of addition must be precisely adjusted tothe specific PZT composition.

SUMMARY OF THE INVENTION

It is an object of the invention to allow for the manufacture of alow-sintering PZT-based piezoceramic having comparable or improvedelectromagnetic properties compared to conventional ceramics of thistype. A sintering temperature in the range of 850° C. to 950° C. may beobtained.

These and other objects of the invention are achieved by a method formanufacturing a low-sintering PZT-based piezoelectric ceramic material,the ions to added being added in the form of powdered oxides and/orpowdered carbonates as starting compounds, mixed together and thencalcined to form the piezoelectric ceramic material, wherein aftercalcining the starting compounds, lithium in salt form is added to themixture in an amount in the range of 0.01 to 0.1 wt. % in relation tothe weight of the PZT ceramic.

Multilayer components having pure Ag internal electrodes may beconstructed using the ceramics manufactured according to the invention.Furthermore, multilayer components having standard internal electrodesbased on AgPd alloys, Cu, or the like may be sintered and manufacturedin a time-saving and cost-effective manner.

DETAILED DESCRIPTION OF THE INVENTION

It is advantageous, if compositions simply doped using rare earthmetals, in particular La or Nd, are used as PZT basic materials.

Additional advantages result from the use of compositions doped usingcombinations of elements selected from the group made up of Ca, La, Nb,Fe, and Cu.

It has been found that controlled addition of lithium in salt form inthe range of 0.01 to 0.1 wt. % in the case of special PZT compositions,for example those proposed in WO 02/055450 A1, is capable of loweringthe sintering temperature of the PZT ceramic by at least 100° C. (from1000° C. to 900° C., for example), the electromechanical propertiesbeing preserved or even improved in relation to comparable PZT ceramicswithout such additives. The amount of additive depends on the PbO excesspercentage and on the selection of the dopant proportions and thus onthe lead lattice vacancies.

PZT-based piezoceramics having a sintering temperature of approximately900° C. and electromechanical properties such as high elongations andlow dielectric constants, for example, which are comparable or evenimproved in relation to the related art, are preferably obtained formanufacturing multilayer actuators for fuel injection systems.

This opens the possibility, for example, of manufacturing actuatorshaving high elongation and other advantageous electromechanicalproperties, which have pure Ag internal electrodes having a relativelylow melting point of approximately 960° C. Actuators having pure Aginternal electrodes have the advantage that they may be sintered in airand thus a higher process complexity may be avoided.

Furthermore, actuators having standard internal electrodes based on AgPdalloys, Cu, or the like may be sintered and manufactured at lowertemperatures and thus in a time-saving and cost-effective manner.

PZT ceramics are usually manufactured from the starting components ZrO₂,PbO, TiO₂, using controlled addition of dopants for precisely settingthe material properties by the known mixed oxide or precursor method. Inthe mixed oxide method, all ions to be added to the piezoelectricceramic element to be manufactured are added as powdered oxides and/orpowdered carbonates, mixed together, and then calcined to form thepiezoelectric ceramic material. The amounts of oxides or carbonates tobe added result from the composition of the piezoelectric ceramicmaterial to be obtained. In contrast, in the precursor or columbitemethod, powdered zirconium dioxide and powdered titanium dioxide areinitially calcined to form Zr_(y)Ti_(l-y)O₂ where 0.5<y<0.55 (in mols);this powder, used as a precursor, is then mixed with powdered oxidesand/or powdered carbonates of the other ions; and this powdered mixtureis then calcined to form the piezoelectric ceramic material in the formof a homogeneous PZT mixed crystal.

Addition of low amounts of lithium in ionic form may lower the sinteringtemperature of the ceramic material obtained by at least approximately100° C. from 1000° C. to 900° C., for example, while preserving orimproving the electromechanical properties.

It has been found that, in the case of simply doped PZT compositions,i.e., in contrast to compositions doped with a plurality of dopantsusing rare earth metals such as La or Nd, the sintering temperature islowered by at least approximately 100° C. Although in these compositionsthe electromechanical properties are not improved, in particular theelongation or the related piezoelectric constant d₃₃ is not increased;however, these properties are not deteriorated.

It has been furthermore found that, in the case of PZT compositionsdoped using combinations of the elements Ca, La, Nb, Fe, Cu, thesintering temperature is not only lowered by at least approximately 100°C., but the electromechanical properties are also improvedsignificantly. The elongation values, which are measured, for example,when a 2 kV/mm electrical field is applied, increase by an order ofmagnitude of up to 25%, as can be seen from the following table.

Elongation in Coercive %o (after Sintering Field polarization) PZTMaterial Temperature [kV/mm] @ 2 kV/mm Nd-doped PZT 1000° C.  1.20 1.07Nd-doped PZT + 900° C. 1.00 1.01 0.05 wt. % Li additive PZT withRB-doping 950° C. 1.35 1.14 PZT with RB-doping + 900° C. 1.11 1.39 0.02wt. % Li additive PZT with RB-doping + 900° C. 0.97 1.40 0.05 wt. % + Liadditive RB doping: Doping as in WO 02/055450.

In addition to the amount of lithium to be added, which is preferably ina range of 0.01 to 0.1 wt. % in relation to the weight of the PZTceramic, the mode of addition is also decisive. Thus, for example, thesintering temperature cannot be lowered if addition takes place as earlyas during homogenization of the starting materials.

It has been shown to be particularly advantageous to add the lithiumcompound after calcining the starting components to achieve the effectaccording to the present invention. If addition takes place beforecalcining, the properties may be improved under certain circumstances,but the sintering temperature is not lowered. It has been observed that,when addition takes place before calcining, the electromagneticproperties are improved, but the sintering temperature cannot belowered. This may be explained by the fact that the relatively volatilelithium compounds evaporate uncontrollably during calcining and thus areno longer available in sufficient quantities to form a low-meltingliquid phase.

Namely, the added lithium compound, together with the existing PbOexcess, forms such a low-melting liquid phase during the sinteringprocess. As a result, the phase of early compacting of the ceramic andthus the entire sintering process is displaced toward lowertemperatures.

In addition, due to the low amount of added lithium compound, a negativeinteraction of the sintering aid with the PZT ceramic and the electrodematerial is minimized, while the positive effect as a sintering aid isachieved. Furthermore, the small amounts of Li⁺ seem to becomeincorporated in the PZT crystal structure and influence the lead latticevacancies of the system in such a way as to favor grain growth and thusto improve elongation.

It is not yet clear what additional effect an assumedly present grainboundary phase may have. Based on the fact that lithium compounds easilyevaporate at these temperatures (approximately 900° C.), the proportionof an otherwise critical secondary phase should be relatively small.

Three possibilities are conceivable:

-   -   a) Lithium is incorporated into the PZT structure (A or B        site)—this has a hardening effect, because it acts as an        acceptor.    -   b) Lithium settles at the grain boundaries—this results in        deterioration of the properties.    -   c) Lithium evaporates—this is advantageous, as long as there is        still sufficient lithium to positively influence the sintering        process.

Addition of excessive amounts of lithium, i.e., more than approx. 0.1wt. %, results in deterioration of the electromagnetic properties. If 1wt. % lithium is added in the form of Li₂CO₃, the measured d₃₃ valuesare a relatively low 286 pC/N (picoCoulomb/Newton) for a relatively highsintering temperature of 950° C., or 352 pC/N for a higher sinteringtemperature of 1050° C.

The addition of lithium in the form of salts, such as Li₂CO₃ or LiNO₃,has been found to be particularly advantageous. Other elements from thegroup of alkaline earth metals such as Na or K, also added as carbonatesor nitrates, are also conceivable as sintering aids.

1. A method for manufacturing a low-sintering PZT-based piezoelectricceramic material having the general formula Pb(Zr_(1-x)Ti_(x))O₃,comprising: mixing together ions added in the form of powdered oxides orpowdered carbonates of at least lead, zirconium and titanium as startingcompounds, and calcining the starting compounds to form thepiezoelectric ceramic material, wherein after calcining the startingcompounds, lithium in salt form is added to the mixture in an amount inthe range of 0.01 to 0.1 wt. % in relation to the weight of the PZTceramic, wherein a sintering temperature in the range of 850° C. to 950°C. is obtained for the mixture of calcined starting compounds and ioniclithium; wherein the starting compound mixture is doped usingcombinations of elements selected from the group consisting of Ca, La,Nb, Fe, and Cu.
 2. A method for manufacturing a low-sintering PZT-basedpiezoelectric ceramic material having the general formulaPb(Zr_(1-x)Ti_(x))O₃, comprising: mixing together ions added in the formof powdered oxides or powdered carbonates of at least lead, zirconiumand titanium as starting compounds, and calcining the starting compoundsto form the piezoelectric ceramic material, wherein after calcining thestarting compounds, lithium in salt form is added to the mixture in anamount in the range of 0.01 to 0.1 wt. % in relation to the weight ofthe PZT ceramic, wherein a sintering temperature in the range of 850° C.to 950° C. is obtained for the mixture of calcined starting compoundsand ionic lithium; wherein lithium is added in the form of Li₂CO₃ orLiNO₃; and wherein the starting compound mixture is doped usingcombinations of elements selected from the group consisting of Ca, La,Nb, Fe, and Cu.